A typical vane-type camshaft phaser for changing the phase relationship between a crankshaft and a camshaft of an internal combustion engine generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is selectively supplied to one of the advance and retard chambers and vacated from the other of the advance and retard chambers in order to rotate the rotor within the stator and thereby change the phase relationship between an engine camshaft and an engine crankshaft. Camshaft phasers also commonly include an intermediate lock pin which selectively prevents relative rotation between the rotor and the stator at an angular position that is intermediate of a full advance and a full retard position. The intermediate lock pin is engaged and disengaged by venting oil from the intermediate lock pin and supplying pressurized oil to the intermediate lock pin respectively.
Some camshaft phasers utilize a bias spring to apply a torque to the rotor in order to urge the rotor to rotate, typically in the advance direction of rotation, to either partially or completely offset the natural retarding torque induced by the overall valve train friction to balance performance times, or to help return the phaser to a default position. The bias spring typically applies a torque to the rotor for the entire range of motion of the rotor 18 within the stator, i.e. between the full retard position and the full advance position, and in the direction toward the full advance position. However, the torque of the bias spring applied to the rotor may make engagement of the intermediate lock pin difficult.
U.S. Pat. No. 7,363,897 to Fischer et al. (Fisher '897) teaches an arrangement to aid in engaging the intermediate lock pin. In this arrangement, the bias spring urges the rotor toward the predetermined aligned position from any position retarded of the predetermined aligned position but does not engage the rotor from any position advanced of the predetermined aligned position. While this arrangement may be effective, the arrangement may increase the axial length of the camshaft phaser which may be undesirable in applications where space for the camshaft phaser is limited.
U.S. Pat. No. 8,127,728 to Fisher et al. (Fisher '728) teaches another arrangement to aid in engaging the intermediate lock pin. In this arrangement, a first bias spring urges the rotor toward the predetermined aligned position from any position retarded of the predetermined aligned position and a second bias spring urges the rotor in the advance direction over the full range of rotation of the rotor. Just as with Fisher '897, the arrangement of Fisher '728 may be effective, but the arrangement may increase the axial length of the camshaft phaser which may be undesirable in applications where space for the camshaft phaser is limited.
What is needed is camshaft phaser which minimizes or eliminates one or more the shortcomings as set forth above.